A newfound plethora of jellies

While many fish are seeing a decline in population, jellyfish are seeing a unprecedented spark in growth.

As the world’s fish populations continue to decline, one sea-faring species, the jellyfish, is flourishing. Pretty as these creatures are, their new found abundance isn’t exactly great news — more like a globular reminder that our oceans’ ecosystems are severely out of whack.

One reason is overfishing. Many fish eat jellyfish or their eggs, so when too many fish are removed by commercial fishing, the jellies have fewer predators. Another problem is pollution. Fertilizers and other pollutants will often find their way to the ocean; they provide extra nutrients to phytoplankton, causing their populations to grow rapidly. When the phytoplankton die, their decomposition depletes oxygen in the water, causing low-oxygen zones that are hazardous to most aquatic creatures — but not to jellies. And climate change is only making things worse, because longer warm seasons are giving jellyfish better conditions in which to reproduce.

With all these jellies around, fish have more competition for food, making it harder for them to bounce back. It might be time to start developing a taste for invertebrates.

Story by Jacqulyn Lane. This article originally appeared in Plenty in October 2006. The story was added to MNN.com in June 2009.